The invention relates to a method for controlling the pressure buildup in an electronically controllable brake system, preferably for use in motor vehicles, including a master brake cylinder, in particular a tandem master brake cylinder (TMC), a vacuum brake booster (booster), at least one additional pressure source for brake force assistance, preferably a hydraulic pump which is drivable by a controlling unit and the pressure of which can be applied to wheel brakes of the vehicle.
The invention also relates to a device for controlling the pressure buildup in an electronically controllable brake system preferably for use in motor vehicles, including a master brake cylinder, a vacuum brake booster (booster), at least one additional pressure source provided with a pump which is drivable by a controlling unit and the pressure of which can be applied to wheel brakes of the vehicle.
The invention further relates to a hydraulic motor vehicle brake system.
Vacuum brake boosters require vacuum supply from the motor in order to assist the pedal force that has to be generated by the driver. Depending on the vacuum supply, defined pedal forces will produce a condition where further increase of the force applied to the actuating unit is only possible by augmenting the pedal force because the vacuum brake booster has reached the maximum possible boosting force. This condition is referred to as operating point of the booster.
Additional brake force assistance is required in case the maximally attainable boosting force is insufficient due to an only weak vacuum supply, which occurs at an increasing rate as regards new engine technology such as gasoline direct injection engines or Diesel engines. One possibility of generating additional brake force or additional brake pressure is the use of ‘active’ hydraulic brake force assistance. This is achieved e.g. by means of a hydraulic pump. The hydraulic pressure, which results in the hydraulic master brake cylinder from the brake force that is introduced by the driver by way of depression of the brake pedal and boosted by means of a vacuum brake booster, is increased in addition by the hydraulic pump. This pump is driven by an electric motor being actuated by an electronic brake control unit.
EP 0 754 607 A1 discloses a brake system with a brake booster and a hydraulic brake control system for vehicles which can generate a pressure that is higher than the pressure introduced. The system is characterized by an evaluation device which identifies when an auxiliary-force to actuating-force ratio (operating point of a booster) falls below a predetermined ratio, by a controlling device which generates a nominal pressure in the wheel brakes by means of the hydraulic brake control system after this ratio has fallen below the predetermined ratio, and by a nominal pressure bringing about, at least in part, a compensation of the effects of an auxiliary force available in relation to the actuating force.
It is a problem in these systems to present an ‘analog’ transition to brake force assistance because only a stepped pressure increase occurs due to valve switching operations or pump operations. The reason is that the valve switching operations and/or pump operations which are necessary for active pressure increase can be executed only when an assessable deviation has developed.
The development of a feeling of a ‘hard’ pedal is not only caused due to ‘waiting for’ a deviation but also occurs with driver specifications (pressure increases) having a raised gradient. The time frame, which is shorter as a result of the increased actuating speed, is not sufficient to compensate the pressure in the vacuum chamber, which rises due to the space decreasing, by way of the available vacuum (low engine vacuum) which is reduced anyway. This means that the pressure difference over the membrane in the vacuum brake booster is lower than would be the case e.g. with a quasi-stationary gradient. Thus, the actuating forces of the brake are too high for the driver.
In addition, a system-induced starting behavior of the pump will also become more conspicuous with increasing depression speed in terms of a time delay as regards the active pressure buildup. The result is that the above-mentioned feeling of a ‘hard pedal’ will be noticed to a stronger degree and influence negatively the feeling of comfort and the possible actuating speed of the driver when braking.
An object of the invention involves providing a method and a device allowing an improved control of the pressure buildup in an electronically controllable brake system.